This disclosure relates generally to light sources and, more particularly, to electrodeless lamps for emitting light in the ultraviolet (UV) and vacuum ultraviolet (VUV) spectra.
Discharge lamps (i.e., bulbs) and particularly electrodeless discharge lamps that contain an emissive material are known. For example, mercury based electrodeless lamps have been in use for many years. See generally Electric Discharge Lamps by Dr. John Waymouth, MIT Press, 1971. The emissive materials employed generally depend on the desired emitted light spectrum. For example, metal halides in combination with halogen doping of electrodeless lamps has been known since the 1960's and can be used to provide various infrared, visible, and ultraviolet containing light spectra. The use of Ba, Na, Ti, In and Cd iodides is disclosed in U.S. Pat. No. 3,234,431. Lanthanides and rare earth metals are also used as dopants in electrodeless bulbs to produce selected spectral emissions. U.S. Pat. No. 3,334,261 lists Y, La, Ce, Nd, Lu, Ho, Th, Pr, Gd, Th, Dy, and Er as dopants for electrodeless bulbs that produce visible light. U.S. Pat. No. 3,947,714 discloses the use of FeI2 as an additive to the constituents of an electrodeless bulb. U.S. Pat. No. 6,157,141 discloses the use of Ga as a dopant in an electrodeless bulb. U.S. Pat. Nos. 5,837,484 and 4,945,290 disclose excimer electrodeless bulbs using noble gases and gas mixtures. U.S. Pat. Nos. 5,504,391 and 5,686,793 disclose excimer electrodeless lamps, which operate at high pressure.
General applications specific to the use of ultraviolet and vacuum ultraviolet light, include, among others, germicidal processes; toxic chemical treatment; curing of inks, coatings, and adhesives; screen printing; CD and DVD replication; label printing; graphic arts; packaging; circuit boards; optical fiber manufacture; and semiconductor manufacturing. However, a problem with the use of electrodeless bulbs operating at these shorter wavelengths is envelope degradation. Photochemical and thermal degradation can occur during use, which changes the spectral output and/or integrity of the lamp. As a result, useful operating lifetimes are deleteriously affected since many end use applications are sensitive to spectral output variation, such as for example, semiconductor fabrication processes that include exposure to ultraviolet and/or vacuum ultraviolet radiation, e.g., charge erasure, curing, photostabilization, surface cleaning, surface modification, oxidation, and the like. The variation in spectral output (both intensities and spectral shape) can affect wafer throughput, which is a significant concern in the semiconductor art.
As is known by those in the semiconductor arts, new processes will require shorter wavelengths as new technology nodes are developed. Light of a given wavelength will have more difficulty penetrating new technology nodes since feature sizes are smaller, metal lines and lines spaces are smaller, and there are generally more metal line layers. Accordingly, as technology nodes advance, there will be a need for shorter wavelength UV and VUV light sources so that the light can penetrate the integrated circuit structure. In a similar fashion, new materials are being developed, such as low-k dielectrics, some of which require wavelengths in the VUV in order to properly treat and/or cure the dielectric. Moreover, repeatability and reproducibility of the semiconductor process is of paramount concern. It is important to maintain processes that are constant from one wafer to the next as well as day-to-day. For UV and VUV processes, it is desirable to have a light source that provides a constant spectrum and intensity.
However, degradation with prior art electrodeless bulbs is particularly acute in the ultraviolet spectrum of about 200 nanometers (nm) to about 280 nm and in the vacuum ultraviolet spectrum of about 122 nm to about 200 nm. Degradation in light intensity can be as much as about 25 percent or more over a one-month period of substantially continuous operation, i.e., including duty cycle this is equivalent to several hundred hours of operation. Degradation is typically manifested by a loss of the shortest wavelengths followed by progressively higher wavelengths. Moreover, because electrodeless bulbs can often operate at very high intensities, the advantages of the higher intensities (shorter process times, for example) are counterbalanced by the faster degradation, since degradation rate is often directly proportional to intensity (photon flux, or number of photons passing through the bulb envelopes).
Accordingly, there is a need for improved electrodeless lamps that emit radiation in the ultraviolet and/or vacuum ultraviolet regions, wherein the electrodeless lamps provide a constant spectrum and intensity for extended periods of time.